Polycrystalline ultra hard material composite abrasive compacts such as cutting elements or shear cutters include a cutting layer such as polycrystalline diamond (“PCD”) or polycrystalline cubic boron nitride (“PCBN”) or thermally stable polycrystalline ultra hard material (“TSP”) formed over a substrate such as a cemented tungsten carbide substrate. Such compacts have well-known applications in the industry. For example, they may be mounted on a rock bit and used for oil, gas drilling and mining operations. These compacts are produced by sintering ultra hard material particles (typically provided in powder form), such as diamond or cubic boron nitride (CBN) particles, over a tungsten carbide substrate at a high pressure and high temperature (HPHT sintering) where the ultra hard material is thermodynamically stable. These temperatures and pressures are typically in the range of 1300° C. to 1600° C. and 5 to 7 GPa, respectively.
Many commercially available polycrystalline ultra hard material layer compacts, such as PCD cutting elements, are formed in accordance with the teachings of U.S. Pat. No. 3,745,623, the contents of which are fully incorporated herein by reference, whereby a relatively small volume of ultra hard particles is sintered in a thin layer of approximately 0.5 to 1.3 mm onto a cemented tungsten carbide substrate. While the teachings of U.S. Pat. No. 3,745,623 utilize a belt press in the disclosed sintering process, it is also known that a cubic press or a Piston-Cylinder (PC) press may also be used.
Generally, to form the polycrystalline ultra hard compact, the ultra hard material along with a cobalt, iron or nickel binder is placed in an enclosure typically referred to as a “can” formed from a refractory metal material such as, for example, niobium, molybdenum or tantalum. The ultra hard material is typically provided in powder form, such as diamond or CBN powder, and may be mixed with a binder or catalyst such as tungsten, cobalt, iron, or nickel, also provided in powder form. A cemented or pre-sintered tungsten carbide substrate is then placed over the ultra hard material in the can. The can is covered with a cover made from the same material as the can. The can and its contents are then placed in a high pressure cell of a high pressure press and are subjected to the HPHT sintering process. The HPHT sintering causes the ultra hard material to convert to a polycrystalline ultra hard material such as PCD or PCBN having intercrystalline bonding between the ultra hard material particles. A catalyst material (such as cobalt) mixed into the ultra hard powder mixture prior to sintering assists in the formation of the polycrystalline structure and facilitates intercrystalline bonding between the ultra hard particles. Alternatively or in addition, catalyst from the substrate infiltrates into the ultra hard layer from the adjacent substrate during sintering and assists in the intercrystalline bonding.
Prior art cutting elements have ultra-hard layers of various thicknesses. Semi-round inserts have a thickness of about 1 mm, and flat chamfered product diamond thickness is up to 2.5 mm. Shear cutters typically have an ultra hard layer that is up to 2 mm thick. A thicker ultra hard material layer is desirable in a compact, as a thicker ultra hard material layer has increased impact and wear resistance. However, an increase in the thickness of the ultra hard material layer may result in delamination of the ultra hard material layer from the substrate due to residual stresses generated at the interface between the ultra hard material and the substrate. During HPHT sintering, the ultra hard material powder shrinks relative to the substrate (which is typically solid or sintered tungsten carbide). In addition, the substrate has a higher coefficient of thermal expansion than does the ultra hard material. As a result, when the substrate and the ultra hard particles are heated during HPHT sintering, the substrate expands to a greater extent than the ultra hard particles. During the cooling down phase of the sintering process, the substrate thermally contracts more than the ultra hard layer does. Also, during HPHT sintering, the ultra hard layer is compacted due to pressure and due to crushing of the particles. The high pressure during HPHT sintering causes both the substrate and the ultra hard layer to become more compact, although, depending on the particle sizes and distributions, they may shrink or consolidate by different amounts. Both of these effects, the particle consolidation and the thermal expansion and cooling, generate residual stresses in the substrate and ultra hard layer near the interface between these two layers.
The relative shrinkage between the ultra hard powder and the substrate increases as the volume and thickness of the ultra hard material layer increases. Consequently, the magnitude of the stresses generated during the HPHT sintering process at the interface, between the ultra hard material and the substrate, increases as the thickness of the ultra hard material layer increases. The increased stresses can cause delamination of the ultra hard material layer from the substrate. As such, the maximum thickness of the sintered ultra hard material layer is limited by the magnitude of these stresses.
Additionally, the shrinkage of the ultra hard material causes a pressure drop in the pressure cell of the high pressure apparatus, which reduces the magnitude of the pressure applied by the press and hinders the HPHT sintering process. If the amount of shrinkage is too high, the press may not be able to apply sufficient pressure to HPHT sinter the material, resulting in a weaker compact. Furthermore, as previously discussed, due to the elastic modulus and coefficient of thermal expansion mismatch between the ultra hard material and the substrate, high residual stresses arise during the cooling down phase of the HPHT sintering process, which ultimately leads to spalling and poor performance of the ultra hard material layer. Consequently, compacts having a better operational performance are desired.